It is known to use buffer layers in organic electronics, such as organic light emitting diodes (OLED), organic photovoltaic cells (OPV cells) or perovskite type solar cells, in order to increase device efficiency and life-time. Such buffer layers comprise metal oxides, such as zinc-, titanium-, tungsten-, nickel-, niobium-oxides, or doped metal oxides, such as Al-doped ZnO (“AZO”) or Cu-doped NiO. Generally, such metal oxides in particulate form are known. Typically, the above named oxidic buffer layers are manufactured by thermal evaporation under high vacuum or by wet-chemical (precursor based) methods, requiring a high temperature annealing step; which is disadvantageous in terms of low-cost, large-area manufacturing processing.
It is also known that organic solar cells (OPV) offer a promising approach for a low-cost and flexible photovoltaic technology with certified efficiencies exceeding 10%. Before widespread commercialization, large area production and stability issues have to be solved. For the reliable large area production with high yield and low shunts, thick, stable, robust and printable buffer layers are a prerequisite.
Generally, such metal oxides in particulate form are known. As discussed above, such oxidic layers are manufactured by thermal evaporation under high vacuum; which is disadvantageous in terms of low-cost, large-area manufacturing processing. Such processes, using comparatively high temperatures, e.g. by including an annealing step, are also disadvantageous in case the layer preceding the buffer layer is temperature sensitive. The present inventors thus identified a need to provide manufacturing processes for buffer layers, particularly metal oxide buffer layers, that are compatible with temperature sensitive layers/materials.
It is also known that Cs2CO3 significantly influences work function of metal oxides in buffer layers. In certain applications, this is considered disadvantageous, as the desired properties of metal oxides interfere with the properties of Cs2CO3. The present inventors thus identified a need to provide metal oxide buffer layers with low or even zero amounts of Cs2CO3.
Luechinger et al. (WO2014/161100) describe organic electronic devices, such as OLEDs and organic solar cells, comprising buffer layers with surface modified metal oxide nanoparticles. Further, the advantages of solution processable buffer layers are outlined. Although simple in manufacturing, through its all-solution-process, the devices disclosed therein show comparatively low performance.
Kim et al. (Adv. Mater., 2014, DOI: 10.1002/adma.201404189) describe perovskite-type organic solar cells comprising NiO and Cu-doped NiO buffer layers. Due to its manufacturing, the buffer layers are dense, i.e. not particulate. The devices show performances exceeding 15% PCE. Nevertheless it is considered disadvantageous that the metal oxide layers are applied by a wet chemical (precursor based) method and thus need to be thermally cured at very high temperatures. Accordingly, these devices are more difficult in manufacturing, as the remaining layers of the solar cells cannot withstand such high temperatures and thus need to be coated after the deposition of the buffer layer.
Liu et al. (Chem. of Mater., 2014, DOI: 10.1021/cm501898y) describe OLEDs comprising NiO hole transport layers. Again, due to its manufacturing, the buffer layers described in this document are dense and not particulate. It is further described that these precursor based layers need to be cured at temperatures of at least 275° C. and even as high as 500° C. Again, this is considered obstructive to the successful production of organic material based electronic devices.
Kim et al (Nanoscale Research Letters 2014, 9, 323) discuss the effect of ZnO:Cs2CO3 on the performance of organic photovoltaics. As stated in that document, the work function of ITO is decreased from 4.7 eV to 3.8 eV due to the modification by Cs2CO3. Such modification of the work function may, depending on the application, be beneficial or disadvantageous.
Yang et al (US2010/0012178) describe solution processable materials for electronic and electro-optic applications. To that end, the electro-optic device comprises an interfacial layer which is a blend of a metal oxide and at least one other material that provides at least one of a decrease in the work function or an increase of electrical conductivity compared to the metal oxide alone. Such other material being present in amounts of at least 10% and up to 120% and thus significantly influence the properties of the metal oxide.
Dong et al (RSC Adv 2014, 4, 60131) discloses the use of Cs2CO3 as surface modification material for hybrid perovskite solar cells.